1. Field of the Invention
The present invention relates to a circuit for driving a solenoid, and more particularly to a solenoid driving circuit for driving solenoids used in solenoid valves, solenoid relays and the like at high speed.
2. Description of the Prior Art
For high speed operation of solenoids, there have been proposed various kinds of circuits for supplying a counterelectromotive force suddenly produced when the current flowing through a choke coil is cut off as the exciting current for the solenoid.
As such a circuit, there is disclosed for example in Japanese Patent Public Disclosure No. 61106/81 an electromagnet driving circuit in which a series circuit of a choke coil and a first switch is connected in parallel with a d.c. power source, one end of the solenoid to be driven is connected with both ends of the choke coil through diodes, and the other end of the solenoid is connected through a switch with one end portion of the d.c. power source.
In this electromagnet driving circuit, the first switch is normally closed so that a steady current is supplied to the solenoid from the d.c. power source through the choke coil, and the switch is normally open. In this condition, when the switch is closed at the same time the first switch is opened, a large counterelectromotive force due to electromagnetic induction is produced in the choke coil. The voltage due to the counterelectromotive force is superposed on the voltage of the d.c. power source to make it possible to provide a suddenly standing-up exciting current to the solenoid.
However, the waveform of the exciting voltage supplied to the solenoid by the conventional driving circuit is extremely sharp, has a large peak value, and is extremely narrow in width. Therefore, with respect to other nearby electronic equipment, it constitutes a high level noise source. Furthermore, the efficiency of the circuit is low since most of the energy is used for the production of noise, and the service life of the switches is short due to the voltage with an extremely large peak value. The efficiency of the circuit is also low since the exciting energy is provided only for a short time. Also, when semiconductor switching elements are used for the switches, it is necessary to use expensive elements which can withstand very high voltage since a voltage with an extremeIy 1arge peak value is produced in this circuit.